During the manufacturing of electrical circuits, such as, e.g., integrated circuit (IC) chips and circuit boards, it becomes necessary to test the electrical characteristics of the electrical circuit at the input/output terminals or contact pads to ensure proper functionality the circuit. In response to the ever increasing complexity and miniaturization of IC-chips and circuit boards, the number and density of circuit bond pads have increased dramatically, requiring very fine pitch testing of the circuits. The current testing methodologies have struggled to adapt to these very fine pitch testing requirements, and in fact, pose a major limitation to the evolution of more complex and smaller IC-chips and circuit boards.
One known method for testing IC-chips (whether in a packaged or bare die form) involves probing the IC-chip contact pads using cantilever probes formed on a probe card. The cantilever probes provide a temporary electrical connection between the IC-chip pads and external test circuitry. The probe card typically includes an insulating substrate, such as a glass filled plastic, with an epoxy ring disposed in the middle. The epoxy ring serves as an electrical and mechanical interface between the cantilever probes, which extend inward from the epoxy ring, and circuit traces, which array outward from the epoxy ring. The arrayed circuit traces terminate at the edges of the probe card to provide a convenient interface for electrical connection to external testing circuitry. The external testing circuitry is specifically designed and programmed to test the IC-chip. The cantilever probes have free ends that are arranged to correspond with the pads of the IC-chip to be tested. Such an arrangement typically takes the form of a hollow or filled square. The ends of the cantilever probes point downward, such that when the probe card is disposed over the IC-chip, the cantilever probe ends contact the respective IC-chip pads.
The main issue with the use of cantilever probes is the consistent and accurate registration of the cantilever probes with the contact pads of each IC-chip. If the ends of the cantilever probes are not in simultaneous and firm contact with the respective contact pads of the IC-chip, inaccuracies in the test results may occur. Simultaneous contact with the multitude of pads, which typically number in the hundreds, requires the ends of the cantilever probes to be vertically registered with the pads of the IC-chip. Thus, if the pads of the IC-chip are in a coplanar relationship, the ends of the cantilever probes should also be in a coplanar relationship. If the pads of the IC-chip are in a non-coplanar relationship (such as may be the case with bare dice), the ends of the cantilever probes should be in a matching non-coplanar relationship. Due to this stringent requirement, the cantilever probes are typically assembled onto the probe card manually. This fact, combined with the already high cost of supplying the cantilever probes (typically, $25 a piece), results in a relative extensive probe card, often exceeding $10,000.
Additionally, due to manufacturing tolerances, there is usually some vertical misregistration between cantilever probes and the respective IC-chip pads. To compensate for this misregistration, a substantial amount of downward force is applied to the cantilever probes, so that all cantilever probes, including those that are misregistered, are in firm contact with the respective IC-chip pads. A greater applied force usually provides a more reliable contact, but can also deform the contact pads in some instances. Additionally, each of these cantilever-type probe cards are typically used to test hundreds of IC-chips. The cantilever probes are, thus, prone to deformation and further misregistration caused by the repeated application of force to the cantilever probes. Thus, to prevent electrical testing failures, the cantilever probes must be manually adjusted periodically, thereby increasing the testing time and cost. In the case of severe misregistration of the cantilever probes, the cantilever-type probe card must typically undergo a substantial amount of rework and repair or be discarded all together, resulting in a great cost burden to the IC-chip industry.
Furthermore, with the increase in density and number of the IC-chip contact pads, it becomes mechanically difficult to arrange a large number of cantilever probes within a limited space of a probe card. This becomes especially difficult when the input/output terminals take the form of bumps disposed not only on, but also disposed inside the periphery of the IC-chip, such as, e.g., with a ball grid array or flip chip. Additionally, pivoting action of the cantilever probes displaces the ends of the cantilever probes in the X-Y plane, thereby requiring a higher degree of pointing accuracy. Cantilever probe technology makes very fine pitch testing of IC's either cost prohibitive or technically unfeasible, and thus the conventional use of cantilever probe technology has now reached its limit.
In response to the afore-mentioned problems with cantilever-type probe cards, the industry has turned to membrane-type probe cards, which typically include a membrane formed of a thin and flexible dielectric material, such as polyimide. Contact bumps are plated onto the membrane in electrical communication with conductive traces, and the conductive traces electrically connect to external test circuitry. The membrane-type probe card is stretched over a solid support. A cushioning member, which absorbs any difference between the heights of the IC-chip bond pads, is disposed between the membrane-type probe card and the support. The membrane-type probe card and frame are made vertically movable so as to bring the contact bumps into elastic contact with the IC-chip bond pads.
Like the cantilever probes described above, a substantial amount of force is required to place the contact bumps in firm contact with the respective IC-chip contact pads. As a result, the contact bumps, which are typically formed of a soft-plated metal, are prone to breaking off or wearing down after several uses. Once a bump is broken or worn, the entire probe card needs to be replaced. In addition, the membrane is repeatedly stressed by large forces, causing the membrane to lose its resiliency.
Another drawback is that bump plating technology is a two-dimensional approach and is limited by current bump-plating dynamics. Thus, membrane-type probe cards cannot be used to test components that do not have coplanar testing surfaces. Still another drawback is that the number of pitch of the contact bumps are limited by the current fine line imaging technology used to form the conductive traces leading to the contact bumps. This is especially true when several rows of contact bumps are desired, thereby requiring interlacing of the conductive traces.
And still another drawback is that conventional membrane-type probe cards pose cleanliness problems when testing IC dice. Membrane-type probe cards typically have a large surface area near the wafer during probing. This area often drapes or droops into contact with wafer areas adjacent to the die under test. Membrane-type probe cards dislodge wafer process particles, which cantilever-type probe cards previously did not touch.
One known method for testing circuit boards involves column testing, i.e., placing a multitude of vertically arranged rigid pins in contact with contact pads on the circuit board. One column testing methodology, called "grid testing," uses a multitude of horizontally arranged plates with holes disposed therethrough. The holes of the top plate are arranged in a roughly pitched grid allowing ease of connection between the top ends of the pins and external test circuitry. The holes of the lower plates are used to thread the pins from the top plate to the finely pitched contact pads of the circuit board. Application of pressure on the top ends of the pins places the bottom ends of the pins in firm contact with the respective contact pads of the circuit board.
The ability to test fine pitch circuit boards using the "grid testing" methodology, however, is limited. Testing of very fine pitch circuit boards necessitates testing with pins having very small diameters. The respective diameters of the plate holes to which the diameters of the pins correspond, can only be drilled so small without exorbitantly increasing the cost of the test assembly. Also, because the pins must be threaded from a rough pitched area to a fine pitched area, the length of the pins must be relatively great, thereby adversely impacting the column strength required by the pins to accurately test the circuit boards.
Even if the circuit to be tested is not finely pitched, the application of pressure on the contact pads of the circuit board from relatively large diameter pins leaves undesirable "witness marks" on the contact pads, which may adversely affect subsequent bonding between a terminal wire and the contact pad. Furthermore, repeated longitudinal movement of the deflected pins through the plate holes wears down the holes, ultimately requiring costly replacement of the plates.
Another column testing methodology, called "dedicated wired spring probe testing," uses a multitude of relatively short pins with springs arranged to correspond to the respective contact pads of the circuit board to be tested. The springs provide resiliency to the pins, allowing the bottom ends of the pins to be placed firmly in contact with the IC-chip bonds pads when pressure is applied to the top ends of the pins. Fine pitch testing, however, requires a large number of costly pins with springs.
Very pitch circuit boards can theoretically be testing using a "flying probe," which involves automatically testing one or more contacts of the circuit board at a time with a moving probe. After each contact pad test, the "flying probe" moves onto the next contact pad. This method, however, is time consuming (taking several hours) and is not always reliable.
Accordingly, a more reliable, efficient, and less expensive apparatus for testing electrical circuits, such as IC-chips and circuit boards, is desired.